1. Field of the Invention
The present invention relates to a method and apparatus for print alignment in an ink jet printer, and, more particularly, to a method and apparatus for bi-directional print alignment in an ink jet printer.
2. Description of the Related Art
When an ink jet printer prints the same horizontal print line (swath) in both left and right-going directions of the carrier, errors are induced due to the travel time of the ink droplets and cock of the carrier due to play in the carrier attachment. As illustrated in FIG. 1, the momentum of a left-going carrier 30 causes ink droplets 32 ejected by a printhead 34 to be carried leftward, resulting in a flight time error 36. Similarly, the momentum of a right-going carrier 30 causes ink droplets 32 ejected by printhead 34 to be carried rightward, resulting in a flight time error 38 (FIG. 2). That is, without alignment, ejecting a vertical column of dots at a given physical encoder marking results in a printed column positioned to the left of the encoder marking location when the carrier is left-going, and results in a printed column positioned to the right of the encoder marking location when the carrier is right-going. In order to eliminate or reduce flight time errors, printers that feature bi-directional print modes must adjust print timing such that the columns of the above example converge on a single location.
Many printers include a manual method of doing xe2x80x9cbi-directional alignmentxe2x80x9d. Usually, this involves the printer driver printing a test page which includes a continuum of alignment possibilities, and having the user manually type in at their personal computer a number or letter representing the pattern with best alignment. From this input, the driver saves timing offsets that allow left and right-going print to align properly.
Automatic bi-directional alignment methods have been featured in a few recent photo-quality ink-jet printers and plotters. Known methods of automatic bi-directional alignment are expensive and include a printed test pattern page scanned by an optical sensor residing on the carrier.
What is needed in the art is a low-cost, simplified bi-directional alignment sensor and, more generally, a simplified bi-directional alignment method.
The present invention provides a low-cost, simple sensor and method for performing bi-directional alignment in an ink jet printer.
The invention comprises, in one form thereof, a printhead alignment sensor for an ink jet printer including two terminals defining a substantially linear gap therebetween. An ink support device supports ink in the gap between the terminals. An electrical measuring device detects a change in an electrical resistance between the terminals when ink is supported in the gap by the ink support device.
The invention comprises, in another form thereof, a method of bi-directionally aligning a printhead in an ink jet printer. A substrate is provided having a target area with a width approximately equal to a width of an ink drop. A carrier of the printhead is moved in a first scan direction from a first location toward the target area. A plurality of aligned ink drops are jetted from the printhead when the carrier is at a first directional jetting location. The aligned ink drops are substantially parallel to the target area. Whether at least one of the ink drops has been jetted onto the target area is sensed. The carrier is returned to the first location. The moving, jetting, sensing and returning steps are repeated until at least one of the ink drops has been jetted onto the target area. Each first directional jetting location is closer to the target area than an immediately preceding first directional jetting location. A first reference jetting location of the carrier is recorded. The first reference jetting location is a location of the carrier when it is sensed that at least one of the ink drops has been jetted onto the target area while the carrier is moving in the first scan direction. The carrier is moved in a second scan direction from a second location toward the target area. The second scan direction is substantially opposite to the first scan direction. A plurality of aligned ink drops are jetted from the printhead when the carrier is at a second directional jetting location. The aligned ink drops are substantially parallel to the target area. Whether at least one of the ink drops has been jetted onto the target area is sensed. The carrier is returned to the second location. The second moving, jetting, sensing and returning steps are repeated until at least one of the ink drops has been jetted onto the target area. Each second directional jetting location is closer to the target area than an immediately preceding second directional jetting location. A second reference jetting location of the carrier is recorded. The second reference jetting location is a location of the carrier when it is sensed that at least one of the ink drops has been jetted onto the target area while the carrier is moving in the second scan direction. The first reference jetting location and the second reference jetting location are used to align ink jetted from the printhead when the carrier is moving in the first scan direction with ink jetted from the printhead when the carrier is moving in the second scan direction.
An advantage of the present invention is that the cost of the sensor is much less than that of a reflective, optical type sensor. The sensing circuit requires just a few low cost components, and the method allows high accuracy of alignment at little cost.
Another advantage is that the method requires only a rough alignment of the sensor in the printer for ease of printer manufacturing assembly.
Yet another advantage is that the method allows alignment to be performed without printing a test page. No user interaction is required. The alignment may take place automatically as soon as a new printhead is identified as having been installed.